1. Field of the Invention
The present invention is related to an electronic circuit, more specifically, to a CMOS/Bipolar/BiCMOS differential to single-ended converter for converting a differential signal to a single-ended signal without the use of operational amplifiers.
2. Description of the Prior Art
Differential type signals, each of which includes one data signal and its inverse, are usually applied in various analog integrated circuits. The differential signals can compensate for environmental noise and interference because the data signal and its inverse are equally affected by the same noise and interference, which is usually called the common mode noise. However, the utilization of the differential signals is not suited to the integrated circuit applications with low pin count. Single-ended signals are preferred to reduce the number of pins in the integrated circuits. Accordingly, a kind of converters for converting a differential signal (two signals) to a single-ended signal, which is usually called a differential to single-ended converter, has been widely developed for facilitating various applications. In addition, the differential to single-ended converter can serve as an output buffer to isolate internal circuitry from output pads or a data buffer to isolate the previous stage from the following stage that requires single-ended input signals.
The conventional differential to single-ended converter is usually implemented by an operational amplifier. FIG. 1 (Prior Art) illustrates a circuit diagram of a conventional differential to single-ended converter. As shown in FIG. 1, the converter includes an operational amplifier 10, resistors R11 and R21 coupled to the inverting input terminal of the operational amplifier 10 and resistors R12 and R22 coupled to the non-inverting input terminal of the operational amplifier 10. The data signal of the differential is coupled to the inverting input terminal of the operational amplifier 10 through resistor R11 and its inverse is coupled to the non-inverting input terminal of the operational amplifier 10 through resistor R12. The single-ended output signal Vout is sent from the output terminal of the operational amplifier 10 coupled to resistor R21.
However, the conventional differential to single-ended converter is not suitable to the integrated circuit application since the embedded operational amplifier usually wastes a lot of chip area and suffers from speed limited.
U.S. Pat. No. 5,432,476 disclosed a differential to single-ended converter without the use of the operational amplifier. FIG. 2 (Prior Art) is a block diagram of the conventional differential to single-ended converter disclosed in the ""473 patent. As shown in FIG. 2, a differential input signal Vin is provided to an input buffer 22. Input buffer 22 provides the data signal and its inverse of the differential signal Vin to V/I converters 24 and 26, respectively. Thus, the output of the V/I converter 24 is provided to a mirror device 28. In addition, a DC level setter 30 establishes a DC voltage level and provides it to a resistive device 32. Finally, the outputs of the mirror device 28, the resistive device 32 and the V/I converter 26 are combined to generate the single-ended output signal Vout. The disclosed differential to single-ended converter is implemented without the use of the operational amplifier, but too many resistors are used for the function implementation.
Therefore, the objective of the present invention is to provide a novel differential to single-ended converter for converting a differential signal to a single-ended signal without the use of the operational amplifier.
The present invention achieves the above-indicated objects by providing a differential to single-ended converter, which is composed of a transconductance amplifier, a current mirror and buffer circuit and a transimpedance amplifier. A differential voltage signal is provided to the inputs of the transconductance stage and converted to a differential current signal. The current mirror and buffer circuit serves as a differential to single-ended current conveyer and isolates the transconductance stage and the following transimpedance stage. Finally, the single-ended current signal is provided to the input of the transimpedance stage and converted to a single-ended voltage signal.
The transconductance stage of the present invention is preferably a source-degenerated amplifier with a degeneration resistor. In addition, the transimpedance stage of the present invention is preferably a shunt feedback amplifier with a shunt feedback resistor. The overall gain of the differential to single-ended converter of the present invention is determined by the ratio of the degeneration resistor value and the shunt feedback resistor value, which facilitates the circuitry design since ratios of resistance values of two resistors can be easily controlled in the semiconductor manufacture process.